The equation of a circle $C$ is $x^2+y^2-6x-7 = 0$. What is its center $(h, k)$ and its radius $r$ ?
Explanation: To find the equation in standard form, complete the square. $(x^2-6x) + (y^2) = 7$ $(x^2-6x+9) + (y^2) = 7 + 9 + 0$ $(x-3)^{2} + y^2 = 16 = 4^2$ Thus, $(h, k) = (3, 0)$ and $r = 4$.